Subradiance

Subradiance is a cooperative phenomenon in quantum optics in which the spontaneous emission of an ensemble of identical emitters is inhibited due to destructive interference among the radiated fields. It is the counterpart to superradiance, where emission is enhanced by constructive interference. In a system of N two-level atoms or other quantum emitters, the emission amplitudes can form collective, or Dicke, states with different coupling to the electromagnetic vacuum. States with antisymmetric or weakly coupled symmetry have suppressed decay rates, sometimes called dark or subradiant states. In the idealized limit of identical coupling and a sample small compared with the wavelength, the fully symmetric (superradiant) state decays rapidly while many subradiant states are effectively dark, with near-zero radiative loss. In real systems, residual decay remains due to finite geometry, position disorder, inhomogeneous broadening, and dipole-dipole interactions.

Subradiance has been explored in cold-atom ensembles, trapped ions, solid-state spin ensembles, and nanophotonic or circuit quantum electrodynamics platforms. Experiments report prolonged lifetimes relative to single-atom decay and controlled suppression of emission by preparing specific collective states or arranging emitters in geometries that favor destructive interference. The phenomenon is described theoretically by a master equation with a collective decay term, where the decay rates are the eigenvalues of the cooperative decay matrix Gamma_{ij}; subradiant states lie in the small-eigenvalue sector.

Potential applications include quantum memories and photonic quantum networks, where suppressed decay helps preserve quantum information and control interactions with light.